Excursion is a large contributor to high output in any driver on the market. Typically, larger excursion (seen visually as how far the cone moves in and out) means more sonic output. However, with excursion come challenges.

As the driver moves outward, the voice coil begins to move out of the gap. This causes a drop in the magnetic force (or BL) and results in a few negative effects, including a large theile/small parameter shift, lower efficiency (magnetic flux decreases), less control over the voice coil (transient response increases, making the driver sound sloppy), and introduces distortion into the output.

Knowing these effects, there was a need to design a parameter to describe the limit of excursion while still remaining “linear”. The idea here was to define a point at which all other parameters remain the same and also consider the limits at which disortion becomes audible. This parameter is referred to as Xmax and, although extremely valuable, is highly controversial. The issue comes back to the actual definition.

Overhang
Traditionally, Xmax has been defined by the overhang method. Overhang is the length of coil in mm that extends past either side of the top plate and is determined by the formula:
Xmax = gap height – [voice coil length/2]
The theory here is that this measurement considers the maximum amount of travel the driver is capable of with the same number of windings (of the voice coil’s wire) in the gap. Once the number of windings decreases, the effects listed above will begin to occur. This is still one of the most commonly used definitions and does work fairly well, but it does not consider several factors, including the behaviour of the magnetic fringe field, any suspension compliance non-linearities, any inductance non-linearities, or InterModulation Distortion. From this lack of refinement, several other definitions have come about.

8mm In the Gap
Digital Designs Audio, famous for their high SPL drivers, use an "8mm in the gap" method. In their opinion, the percentage of coil still in the gap is what matters, rather than the overhang. For DD Audio, this is an important method to consider. They typically use tall top plates and, as such, have very long magnetic gaps that cannot be considered by the traditional overhang method of measurement. Using the overhang method has consistently given DD Audio very low numbers, although most newer and more accurate methods do as well. The issue here is that once the coil begins to move out of the gap, there is a drop in BL, whether there are 8mm of coil in the gap or not. Using DD’s measurement method, this drop in BL would not be accounted for. Due to the large gap that DD uses, this drop is not as drastic as others, but it is a drop in BL regardless. This measurement method also does not cover any of the other issues mentioned with the overhang method.

DUMAX and Klippel
There are two revolutionary measurement systems now available: the DUMAX from Dave Clark and DLC Designs, as well as the Klippel, developed by Dr. Wolfgang Klippel. These measurement systems compare BL with respect to x(mm of excursion). They also consider suspension compliance (Cms) as this will create many of the same issues that a shift in BL will. This is done as 1/Cms (which=Kms) with respect to x(mm of excursion). Both of these demonstrate through a graph the measurement of BL and Cms through both the positive and negative strokes. An example of each graph can be found below:
BL from Klippel
Kms (1/Cms) from Klippel
BL and Kms from DUMAX (scroll down to the graph section)

DLC defines Xmax as the smaller of 1: half of the peak to peak excursion over which BL retains 70.7% of it’s rest position value or 2: half of the peak to peak excursion over which Cms retains 25% of it’s rest position value. These values represent approximately 10% distortion, which is often considered the point of audibility. By using this method, we consider the

 

nice find dude.....sticky it!